In numerous mechanical design applications, including the design of isolators and of bi-stable elements, negative stiffness elements are potentially useful. For example, a soft spring may improve the performance of an isolator using such a spring, and a soft spring may be fabricated by combining a positive stiffness spring with a negative stiffness spring. Negative stiffness elements also may display bi-stable behavior, if the range of motion over which the element exhibits negative stiffness is less than the total range of motion of the element.
A negative torsional spring may have applications in these categories, for example being useful in fabricating a drive shaft isolator with a low spring constant, or in fabricating a hinge with two stable positions, which may be used, for example to provide two stable positions for a component on a satellite, e.g., stowed for launch, and deployed. Negative torsional stiffness elements may be constructed by coupling a linear, or axial, negative stiffness element to a rotating element, e.g., using a conversion mechanism, such as a rack and pinion arrangement, to convert rotational motion to translational motion, but such solutions add mass and may be subject to wear. Thus, there is a need for an assembly with negative torsional stiffness which does not rely on a conversion mechanism to convert rotational motion to translational motion.